The Crucible of Souls, located on an asteroid, was an invention of the Doom Coalition. It was created in order to convert the life energy of the people at the end of the universe into regeneration energy to make the Time Lords immortal.
The Nine intended to use it to give himself immortality, and become "the Numberless" or "the Eternity". (AUDIO: The Crucible of Souls)
While the Crucible was being decommissioned, it was attacked by the Eleven and the Ravenous, who had stolen the Matrix print of Artron, an ancient Gallifreyan scientist. Using Artron's knowledge the Eleven was able to reconfigure the Crucible so that, rather than absorbing energy from the rest of the universe, it could grant the gift of regeneration to the population of the universe, making them a new source of food for the Ravenous. After three incarnations of the Master brought the still-living Artron to the Crucible, the Doctor, Liv and Helen were able to convince him to restore the Ravenous to their original nature while the Masters reconfigured the crucible to remove the ability to regenerate from the rest of the universe. (AUDIO: Day of the Master)